The invention relates to a rear axle suspension for large vehicles, and it especially relates to a device designed to increase traction of vehicles with two rear axles when they begin moving.
FIG. 15 is a diagram of a vehicle with two rear axles. A large vehicle (1) has a front axle (21) at a front of a frame (10), which supports front tires (21a), and two axles (31, 32) at a rear, which support rear tires (31a, 32a), respectively.
The rear axles (31, 32) are supported by the frame (10) via a suspension (40), and the cargo (50) is loaded on the frame (10).
The overall weight (WG) of the vehicle (1), including the cargo (50), is distributed to load (WF) applied to the front tires (21a) and load (WG) applied to the rear tires (31a, 32a), which are supported by the road surface (G).
By providing the two rear axles (31, 32), the weight (WR) supported at the rear is divided into weight (WRF) supported by the front rear axle (31) and weight (WRR) supported by the rear rear axle (32). Therefore, the structure of the vehicle allows larger cargo load while reducing the weight per axle.
It is common to structure this type of a two-rear axle vehicle with a drive supplied to two tires of the total six, so called 6.times.2 vehicle, by simplifying the power transmission system such that the front rear axle (31) is a drive axle that conveys driving force from the engine and the rear rear axle (32) is non-powered, free-rolling driven axle.
Drive axle tires (31a) on the 6.times.2 vehicle are required to generate sufficient driving force. Thus, the weight (WRF) on the tires (31a) must receive a sufficient load for the tires to generate a driving force against the road surface.
When the cargo is full, weight (WRF) on the drive axle (31) is sufficiently large to enable the tires (31a) to generate the necessary driving force. If the cargo is empty, however, the tires (31a) may not be able to produce sufficient driving force against the road surface (G) because the weight (WRF) on the drive axle (31) is insufficient. That is especially true when the vehicle begins to move or travel on the road surface with a small coefficient of friction, by which the tires (31a) may slip and thus cannot drive.
Japanese Utility Model Laid-Open No. 57-96081 (1982) proposes a device for vehicles that support two rear axles with a walking beam suspension, which lifts the driven tires by a hydraulic cylinder when the cargo is empty.
Air springs may be used instead of plate springs for the suspension of this type of large vehicle, and Japanese Utility Model Laid-Open No. 5-32013 (1993) discloses a suspension using air springs.
As indicated in the above mentioned Japanese Utility Model Laid-Open No. 57-96081, the 6.times.2 vehicle is driven as a 4.times.2 vehicle by lifting the rear driven tires when the cargo is empty, so that the vehicle generates sufficient driving force and has a desirable effect on fuel consumption, as well.
However, such a device as described above lifts the driven tires by means of a hydraulic cylinder. The lifted tires are stationary and supported at a certain ground clearance. When the vehicle is driven under these conditions, the driven tires may come in contact with the road surface on bumpy roads. Since the driven tires are stationary, they receive a large shock at the moment of contact with the road surface and the tires are damaged.
When a walking beam suspension is used, if the driven axle is lifted, the plate spring flexes towards the frame, which, in turn, reduces suspension performance.
The object of the invention is to provide a device that helps a vehicle to start moving by increasing the ground pressure through use of a trunnion suspension formed of plate spring and torque rods, and an air spring, and it also provides a structure for a reasonable air spring mounting.